That the Fauna of Europe in General, and of Britain in Particular, Gives A
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The atyid prawns of Dominica Item Type book_section Authors Fryer, G. Publisher Freshwater Biological Association Download date 25/09/2021 10:40:53 Link to Item http://hdl.handle.net/1834/22690 48 FORTY-FIFTH ANNUAL REPORT THE ATYID PRAWNS OF DOMINICA G. FRYER That the fauna of Europe in general, and of Britain in particular, gives a very imperfect idea of the importance of several groups of freshwater animals is perhaps insufficiently appreciated by European workers. An excellent example of such faunal poverty is provided by the decapod crustaceans - the group that includes crabs, crayfishes and prawns. Of this group there is only one truly native British species, the common crayfish (Austropotamobius pallipes). In warmer regions, and especially in the tropics, the situation is often different. Here prawns and crabs are some- times important constituents of the freshwater fauna. In few places can this be more clearly seen than in the West Indian island of Dominica. Here, where a copious rainfall is reflected by much rich forest and many perennial streams, the number of species of prawns greatly outnumbers that of fishes, which they replace as the dominant large animals of flowing water. The streams also harbour aquatic and semi-aquatic crabs thus emphasizing crustacean dominance. Of the prawns six species belong to the family Atyidae (Fig. 1) and five to the Palaemonidae. It is the former family that concerns us here. Atyids, which students of the Crustacea regard as primitive prawns, have a circumtropical distribution but additionally some have penetrated PLATE 3 Atya innocous advancing over a stone and collecting food by sweeping as it does into temperate regions, sometimes in surface waters, sometimes as so. The white particles are a proprietary breakfast food laid as bait. (See Plate 4.) cavernicolous forms. An intriguing aspect of their distribution is that, although they are, and have long been, freshwater animals - fossils are known from freshwater deposits of Cretaceous age, and they have produced endemic genera in Lake Tanganyika — they have succeeded in colonizing many oceanic islands. The West Indies is an area of particular interest to the student of the family as representatives of several genera sometimes occur on one island. For its size Dominica is particularly rich in this respect. Especially interesting is the fact that its fauna includes not only the most primitive living atyid (a West Indian endemic, Xiphocaris elongata) but also two representatives of what is almost certainly the most advanced genus, Atya. Each of the other three species belongs to a separate genus. All Dominican atyids are detritus feeders. Their food comes from the forest, leaves from which fall continuously into the streams and there break down into particles sufficiently small to be collected by the specialized food-gathering devices which they employ. If the forest is destroyed the atyids will disappear, either as a result of the lack of food or by scouring and drying up of the streams. The primitive Xiphocaris, which reaches a length of 7 cm if one includes the long rostrum, is a lightly-built, transparent prawn with a well-defined ecological niche. It frequents quiet pools in streams over whose bottoms it tiptoes delicately, picking up individual small particles of detritus with its two pairs of chelipeds ('nippers') that are specialized for this purpose. It is DIRECTOR'S REPORT 49 PLATE 4 (above) As Plate 3. The animal has advanced further and its chelipeds are shown at different phases of the cycle of movement. (below) Atya innocous facing a current with all its cheliped bristles spread to form four contiguous filtering baskets that passively collect suspended food particles. FIG. 1. The atyid prawns of Dominica. also an efficient swimmer, and employs a very spectacular escape mechanism. When approached it leaps backwards by flexing its abdomen with such force as to propel it out of the water and through the air for con siderable distances. On alighting it may make one or more additional leaps, presumably by further flexions of the abdomen, before disappearing beneath the surface. In its method of food collection it differs from all other atyids in that its chelipeds are used directly for seizing food particles, as in the familiar crabs and crayfishes, whereas its relatives employ rows of highly specialized cheliped bristles. Although primitive, Xiphocaris employs complex machinery for dealing with the collected food in its passage from the chelipeds, via the mouthparts, to the mouth. A second complicated mass of apparatus located in the fore-gut is then used for sorting and processing the collected material. All other Dominican atyids are crawlers rather than swimmers, and all have an elaborate method of food collection, again employing the chelipeds, but in a different way from Xiphocaris. In these species each 'finger' of the pincer of the cheliped, itself highly modified, is fringed by 50 FORTY-FIFTH ANNUAL REPORT DIRECTOR'S REPORT 51 rows of elaborate bristles whose form, number and arrangement differ Both the Dominican species of Atya frequently filter in this way, often from species to species. In all cases these bristles are expanded into an for many hours at a time, but aquarium observations suggest that A. opposing pair of fans as the cheliped is extended, and can then be placed scabra does so more persistently than A. innocous and that, while the against a stone or other object and brought together. In so doing each fan latter regularly sweeps up particles, the former seldom does so. This sweeps up small particles and when the two come together, closing as they difference is reflected in slight differences in the nature of the bristles. A do so, they enclose an accumulation of such material. This can then be few of those of A. innocous have fine teeth, detectable only under the passed to the mouthparts. microscope, which will help it to sweep up particles, but such are lacking Such is the basic outline of the process. Many refinements are possible in A. scabra. This difference helps us to understand the ecological and have been exploited. The way in which the fans are expanded and preferences of the two species and may go some way to explaining why A. closed also demands attention. Clearly, simple bristles will be less effective innocous is much more common than A. scabra. Its greater efficiency as a at sweeping than those which are modified by the development of ridges or scraper may give it an advantage. side bristles so it is not surprising to find many such modifications nor to Another Dominican atyid is Micratya poeyi which, with a maximum find that different species have specialized in different ways to enable them length of only about 2.3 cm, is in many respects a miniature version of to exploit different habitats and niches. For example, the small Jonga Atya. This is an extremely abundant species and, like Atya, can both serrei (generally less than 2.5 cm in length) habitually lives in slow-flowing scrape and filter passively. The least known species is Potimirim glabra water, usually near stream mouths, not because it needs brackish water whose cheliped bristles are specialized for sweeping and show few signs of but because in the mountainous Dominica it is only here that such slow being used for passive filtration. flow is to be found. Here it scrapes both hard objects on the bottom and The food collecting mechanism thus briefly described presents many marginal vegetation and its bristles have become highly specialized as problems to the zoologist. Particularly intriguing is the way in which the scrapers and brushes. Furthermore they are arranged in arcs so that, as fans of bristles are expanded. Examination of the way in which the bristles the pincers close and the two fans sweep towards each other, the inner arcs are inserted on the cheliped segments reveals that no muscles whatsoever of beautifully formed scrapers pass over the substratum first. These are are attached to any of them - yet they are clearly under the precise control followed by larger brushes that sweep up any material that is dislodged but of the animal. The secret is revealed by the way in which they are attached not passed on directly by the scrapers. Thus are surfaces efficiently swept. to the hollow cheliped segments. Each bristle is tubular and sits over a hole Specialized in a different way but employing similar principles are the in the cheliped cuticle (Fig. 2). Thus its cavity is in direct continuity with two species of Atya- A. innocous (to 12 cm+) which is very common, and the fluid-filled cavity of the cheliped, itself continuous with the haemocoele A. scabra (to at least 10 cm) which is much rarer. Both have extremely of the trunk. Further, each bristle is beautifully pivoted so that it can long cheliped bristles, so much so that the tip of a closed cheliped swing freely, but only in one direction. Apart from the pivot the gap resembles the business end of an artist's paint brush. In order to permit between the bristle and the cheliped cuticle is spanned by cuticle, thin in this bunching together of the bristles there have had to be fundamental part but, opposite the pivot, thrown into a fold. Now, if haemocoelic changes in the organization of the distal segments of the cheliped such as pressure is locally increased, pressure in each bristle increases and, because are not to be found in other decapods but which need not concern us here. it is pivoted, it swings in a predetermined direction until it can go no When these bristles are expanded they make very large fans which can be further.